A method is known to reduce the surface reflection on optical parts such as lenses and prisms in which some layers of thin films are formed on the surfaces of the optical parts by vacuum evaporation or the like.
FIG. 33 shows an example of a conventional four-layer antireflection coating (first prior art). In FIG. 33, the conventional antireflection coating comprises from the substrate side: a first layer of SiO.sub.2 (silicon dioxide, refractive index n1=1.47, thickness d1=42 nm); a second layer of Al.sub.2 O.sub.3 (alumina, refractive index n2=1.62, thickness d2=122 nm); a third layer of Ta.sub.2 O.sub.5 (tantalum oxide, refractive index n3=2.10, thickness d3=141 nm); and a fourth layer of MgF.sub.2 (magnesium fluoride, refractive index n4=1.385, thickness d4=107 nm). The design dominant wavelength .lambda..sub.0 is 550 nm, and nd represents the optical thickness (in nm). At this time, a low reflectance is obtained.
As another example of the conventional four-layer antireflection coating, FIG. 34 shows an antireflection coating described in Japanese Laid-Open Patent Application number 62-42101 (second prior art). In FIG. 34, the antireflection coating of this prior art comprises from the substrate side: a first layer of SiO.sub.2 (thickness d1=94 nm); a second layer of Al.sub.2 O.sub.3 (thickness d2=85 nm); a third layer of Ta.sub.2 O.sub.5 (thickness d3=131 nm); and a fourth layer of MgF.sub.2 (thickness d4=99 nm) At this time, a low reflectance is obtained in a wide band.
As an example of a conventional five-layer antireflection coating, FIG. 35 shows an antireflection coating described in Japanese Laid-Open Patent Application No. 5-2101 (third prior art). In FIG. 35, the antireflection coating of this prior art comprises from the substrate side: a first layer of MgF.sub.2 (magnesium fluoride, refractive index n1=1.385, thickness d1=64 nm); a second layer of ZrTiO.sub.4 (titanium zirconium oxide, refractive index n2=2.1, thickness d2=16 nm); a third layer of SiO.sub.2 (silicon dioxide, refractive index n3=1.47, thickness d3=54 nm); a fourth layer of ZrTiO.sub.4 (refractive index n4=2.1, thickness d4=139 nm); and a fifth layer of MgF.sub.2 (refractive index n5=1.385, thickness d5=99 nm). The design dominant wavelength .lambda..sub.0 is 550 nm, and nd represents the optical thickness (in nm). At this time, a low reflectance is obtained.
As another example of the conventional five-layer antireflection coating, FIG. 36 shows an antireflection coating disclosed in Japanese Laid-Open Patent Application number 10-39105 (fourth prior art). In FIG. 36, the antireflection coating of this prior art comprises from the substrate side: a first layer of Al.sub.2 O.sub.3 (alumina, refractive index n1=1.62, thickness d1=67 nm); a second layer of ZrTiO.sub.4 (refractive index n2=2.1, thickness d2=10 nm); a third layer of Al.sub.2 O.sub.3 (refractive index n3=1.62, thickness d3=17 nm); a fourth layer of ZrTiO.sub.4 (refractive index n4=2.1, thickness d4=100 nm); a fifth layer of MgF.sub.2 (refractive index n5=1.385, thickness d5=100 nm). At this time, a low reflectance is obtained in the entire visible wavelength range.
Generally, when the incident angle of the light incident on an optical part is large, the polarized components are separated, and the s-polarized component of the light is higher in reflectance than the p-polarized component of the light. For this reason, the s-polarized component of the light is mainly used to thereby decrease the reduction in the quantity of projected light, particularly, in liquid crystal projectors and the like having illumination optical systems using polarized light.
However, it is necessary that reflectance be low in the antireflection coating. According to the antireflection coatings of the prior arts, although reflectance is low for both the p- and the s-polarized components in a range where the incident angle is small, the reflectance Rs of the light of the s-polarized component is high in the neighborhood of an incident angle of 45 degrees because of the separation of the polarized components. FIGS. 37 to 40 show reflectance characteristics of the antireflection coatings of the structures of FIGS. 33 to 36 at an incident angle of 45 degrees. In FIGS. 33 to 36, the vertical axes represent the reflectance and the horizontal axes represent the wavelength.
According to FIGS. 33 to 36, in the neighborhood of the design wavelength .lambda..sub.0 (550 nm), although the reflectance Rp of the light of the p-polarized component is low, the reflectance Rs of the light of the s-polarized component is high. Therefore, when the light of the s-polarized component is made incident at an incident angle of 45 degrees on a filter or the like having any one of the antireflection coatings applied to the reverse surface thereof, a double image is formed by the light reflected at the surface of the filter and the light transmitted by the filter and reflected at the antireflection coating.